The use of lasers for scribing the thin layers found in solar panels to create and interconnect sub cells been well known for many years. The technology consists of laying down a thin layer of the lower electrode, often ITO, on a glass plate and laser scribing lines at typically 5-10 mm intervals to separate the ITO layer into electrically isolated regions. The electricity generating layer such as amorphous silicon is then applied over the whole area and a laser is used again to scribe lines in this layer parallel to and as close as possible to the initial scribes in the first layer. A third, top layer, often aluminium, is then applied and a laser beam is used for the third time to scribe lines in this layer as close to and parallel to the other lines to break the electrical continuity.
By this method an electrical series connection is made between all the cells in the panel so that the voltage generated by the whole panel is given by the product of the potential formed within each cell and the number of cells. Typically panels are divided up into 50-100 cells so that overall panel output voltages are in the 50 volt range. JP10209475 gives a thorough description of the standard laser processes used.
As well as ITO/Silicon/Aluminium structures many other materials can be used to make solar panels. Other equally effective devices are made based on Cadmium Teluride (CdTe), Copper-Indium-diselenide (CIS) and crystalline silicon on glass (CSG). In all cases lasers are used to scribe some or all of the layers involved.
The laser beams that are used to scribe individual layers are sometimes applied from the coated side of the glass sheet but can also be applied from the opposite side in which case the beams pass through the glass before interacting with the film. The lasers used generally operate in the infra-red (IR) region of the spectrum but lasers operating at the 2nd harmonic wavelength (532 mm) are also widely used. Even UV lasers are sometimes used. The lasers are generally pulsed with pulse lengths in the range of a few to several 100 nanoseconds and operate at pulse repetition rates in the range of a few kHz to few MHz.
In some cases solar panels are made on non-transparent substrates such as metal sheets. In this case irradiation through the substrate is not possible so all scribing processes require beams incident from the coated side. In some other cases solar panels are fabricated on flexible substrates such as thin metal or polymer sheets. In the former case irradiation from only the coated side is possible. In the latter case irradiation from the coated side or through the substrate are both possible.
The common characteristics of all these devices is that multiple scribes each up to one or more metres in length have to be created in order to divide up each layer on a panel. Hence total scribe lengths per layer up to well over 100 m often need to be made by solar panel process tools in acceptable panel process times. These are generally less than 2 minutes. This means that laser scribing rates up to many metres per second are required.
Laser tools have already been built to achieve this. In some cases the tools have stationary optics which means the panel has to be moved very rapidly. To avoid excessive panel speed multiple stationary parallel optics heads are often used. As an example of this a panel with dimension of about 1.1×1.1 m requiring 160 separate scribes can be processed with 8 parallel beams in under 100 seconds with the panel moving at a maximum speed of less than 300 mm/sec. Such an approach is acceptable but complex in terms of the number of optics heads and laser beam splitting and balancing requirements. Having large heavy stage systems moving repetitively backwards and forwards at high speed also leads to unreliability.
Another approach has been to use a single beam to scribe all the lines but cause the beam to move at high speed using a galvanometer driven mirror scanner system. U.S. patent Application Publication No. US2003/0209527A1 describes such a case. A scanner system is used to move the laser beam across the full width of a 600 mm wide panel at speeds up to 4 metres/sec while the panel is moved in the orthogonal direction past the scanner unit.
This invention is effective in that no high speed panel motion is required and that only one beam is used but has the problem that to cover the full width of the panel a scanner lens with a large scan field has to be used. This usually means that the lens has a relatively long focal length. It is also often necessary to use a scanner system with a 3rd axis to dynamically adjust the beam expansion during each scan in order to maintain focus over the full panel width. This adds complexity to the control system. The long focal length of the scan lens required leads to limitations to the minimum size of the focal spot that can be created and hence the scribe width that can be made is not as narrow as desired. It also leads to difficulties placing the scribes accurately as the positioning errors associated with scanner systems scale with the lens focal length. Both of these are major problems as ideal scribes should be as narrow in width as possible and successive scribes should be as close together as possible as the area between the three scribes does not generate electricity and therefore needs to be minimised.
US2003/0209527A1 also introduces the concept of continuous motion of the panel during scribing leading to what is termed a ‘bow tie configuration’. Such a method is effective compared to step and scan processing and can be used readily where the position of the scribes is not critical. However, in the secondary and tertiary panel scribing processes where scribes need to be made very close to previous scribes, the bow tie configuration is difficult to implement.
In the situation where scribes need to be placed reliably very close to existing scribes because of panel distortions and size changes during manufacture it is necessary to measure the position of previous scribes and compensate by adjusting the scanner motion to maintain accurate relative positioning. A global measurement of overall panel expansion or shrinkage is readily made by measuring the position of the first and last scribes on a panel after loading. This data can be used to correct for these global changes by adjusting the parameters that control the panel motion through the tool or even correct for minor tilt to the lines. However, simple global distortion correction is not sufficient to allow close and accurate placement of scribes since scribe pitch can become irregular due to errors on the tool that make the first scribe or errors introduced during subsequent panel processing.
The invention described here uses a bow-tie configuration of the type described in US2003/0209527A1 but implements it in a way that seeks to overcome all the limitations described and allows a dynamic alignment system to be used that ensures that all scribes are accurately placed with respect to previous scribes. We call our scanning method ‘bow tie scanning’ (‘BTS’) and our alignment system ‘dynamic scribe alignment’ (‘DSA’)